V O L U M E 2 6 , N O . 6, J U N E 1 9 5 4
1053
to its solubility in mineral acids. At a p H between 3 to 5.5, precipitation n-as elow, and irregular results R ere obtained. I n the p H range betrveen 5.5 and 6.2, accurate and reproducible values were found. Above pH 6.5 values again became irregular and high, presumablv owing to the presence of carbon dioxide from the air, resulting in the precipitation of barium carbonat_. hfter several determinations the solutions with high p H values had a poisoning effect on the silver-silver chloride electrode Several buffei systems were investigated for the purpose of maintaining the p H within the designated range of 5.5 to 6.2. These included acetic acid-sodium acetate, malonic acid-sodium malonate, and succinic acid-sodium succinate. -411 these buffers, even in very small amounts, completelv inhibited the reaction tietween barium and chromate ions. This was apparent from the fact that in addition to the absence of an observable precipitate of barium chromate, the current increased after the first increment of chromate had been added. I n view of this unpredicted phenomenon, the pH thereafter was adjusted to the proper value without the use of buffers bv simplv adding acid or alkali until the solution was orange to methyl red. Effect of Alcohol Concentration. Because the solubility of barium rhromate is noticeably affected by the presence of in-
different salts, Ilolthoff and Gregor ( 2 ) carried out the titration in the preience of alcohol. I n order to ascertain the minimum concentration of ethyl alcohol that would be required to yield reliable results under the conditions of the determination, this factor was studied. The results obtained by titrating synthetic media similar to those used for the p H study but containing increasing concentrations of ethyl alcohol are illustrated in Figure 2. As seen in the figure, the solubility of barium chromate is appreciable when the alcohol concentration is below 50%, making the base line, and therefore the end point, uncertain. Consequently, i t is recommended that the ethyl alcohol concentration be at least 5070. LITERATURE CITED
(1)
Hegrorsk$, J., and Berezicky, S.,Collection Czechosloc. Chem.
Communs., 1, 19 (1929). ( 2 ) Kolthoff, I. AI., and Gregor, H. P., - ~ N . A L .CHEM., 20, 541 (1948). (3) Kolthoff, I. AI., and P a n , Y. D., J . .4m. Chem. Soc., 62, 3332 (1940).
Majer, V., 2. Elektrochent., 42, 120, 123 (1936). ( 5 ) Spalenka. A f . , Collection Czechosloc. Chem. Commz~ns.,11, 146
(4)
(1939). R E C E I V Efor D review October 1 , 1953.
Accepted February 5 , 1954.
Determination of Formic Acid by Oxidation with lead Tetraacetate A. S. PERLIN Division o f A p p l i e d Biology, National Research Laboratories, Ottawa, Canada
S
LOIT oxidation of formic acid to carbon dioxide by lead tetraacetate in aqueous acetic acid was originally reported by Grosheintz (4). Alore recently, Mosher and Kehr ( 5 ) have described the reaction with glacial acetic acid as solvent. The oxidation is greatly accelerated by potassium acetate, and the consequent enhanced rate of evolution of carbon dioxide makes the reaction suitable for manometric measurement of formic acid production during lead tetraacetate oxidations of carbohydrates (6). T h e present paper proposes t h a t the accelerated reaction may have M ider application for the determination of formic acid. I n some respects, it offers advantages over the usual analysis by oxidation with mercuric chloride (1, 2 ) . For example, the lead tetraacetate oxidation provides a simple, accurate, volumetric analysis (Table I, procedure 1) which is much more rapid and less tedious than the gravimetric procedure of the latter method. Further, 4hlBn and Samuelson ( 1 ) have shon-n t h a t
Table I.
Determination of Formic Acid
formaldehydp interferes with the mercuric chloride method and must first be separated from the formic acid-e.g., in the analysis of sulfite waste liquors. I n contrast, formaldehyde has no effect on the determination of formic acid by oxidation with lead tetraacetate (Table I). The lead tetraacetate procedure can also be used for determinations in the presence of other acids such as acetic, propionic, and succinic (Table I), and may therefore find application in the analysis of fermentation liquors. Formic acid may be determined not only by titration of the amount of reduced lead tetraacetate but also by measuring t,he amount of carbon dioxide evolved. T h e latter procedure is especially useful in the presence of substances-e.g., glycolswhich consume lead tetraacetate but do not yield carbon dioxide or formic acid. The Warburg respiromet'er is suitable for the micro range (6) (Table I, procedure 2.i), or a gas adsorption train for larger quantities of formic acid (Table I, procedure 2B). Procedures 2 d and 2B are suited also to the determination of compounds, such as glycerol and osalic acid, which yield formic acid or carbon dioxide vihen oxidized with lead tetraacetate. EXPERIXIENTA L ~
Procedure 1
2A
2B
Other Compounds .4dded, Added. ME. hlg. ... 1 05 2 14 2.1.4 2 40 (Formaldehyde) 2.11 5.0 (Propionic acid) 0.5 (Succinic acid) 4.22
HCOOHa
0.080 0.16 0.33a 21.5
0:k3b (Formaldehyde)
Found. I l g . 1.06 2.12 2 13
Reaction Time, Min. 20 20 20
2.11
20
4 18
20
0 077 0 10 0 33
30 30 30
21.3 21.4 32 1 32.7 BY alkali titration of a stock aqueous solution of formic acid. Produced i n siiu by oxidation of 0.66 mg. of glycerol. 32.3
a
...
HCOOH
75
90
(io
7.5
~~
Reagents. Formic acid, reagent grade. Glacial acetic acid, reagent grade. Lead tetraacetate. A satisfactory method of preparation is given by Vogel (8). Anhydrous potassium acetate, reagent grade. Stopping solution: 10 grams of potassium iodide and 50 grams of sodium acetate dissolved in 100 ml. of water (3). ~, Standard 0 . 0 2 5 sodium thiosulfate. Procedure 1. Five milliliters of a solution of formic acid (1 to 5 mg.) in 80% acetic acid are added to 5 ml. of oxidizing solution, consisting of 100 mg. each of lead tetraacetate and potassium acetate in glacial acetic acid, in a glass-stoppered flask. X blank is prepared from 5 ml. of 807, acetic acid and 5 ml. of oxidizing solution. When the reaction has proceeded at room temperature (25" to 27" C.) for 20 to 30 minute$, 10 ml. of stopping solution is added. The yellow precipitate of lead iodide which appears may be dissolved b v addition of water and the iodine is titrated with standard t h i o d f a t e to a starch end point. The titration is equally satisfactory with the lead iodide present, but a pea-green color is given by the starch-iodine complex instead of the normal
1054
ANALYTICAL CHEMISTRY
blue, and the end point is then indicated by the sudden reappearI ance of the bright yellow of the lead iodide. Procedure 2A. A conventional constant-volume type of Warburg respirometer is used for measurement of carbon dioxide produced. Detailed descriptions of the apparatus and its manipulation are given by Umbreit et d.( 7 ) . The reaction temperature in the author’s experiments was 27” C. One milliliter of oxidizing solution (10 mg. each of lead tetraacetate and potassium acetate in 90% acetic acid) is added t o the vessel and 0.2 ml. of the formic acid solution (0.1t o 0.5 mg. of formic acid in 90% acetic acid) is placed in the side arm. A blank is run simultaneously with 1 ml. of oxidizing solution in the vessel and 0.2 ml. of 90% acetic acid in the side arm. The vessels are equilibrated for 10 t o 15 minutes, the contents are mixed, and the change in pressure is observed until a constant value is attained (20 to 25 minutes). The apparatus is calibrated in the same manner by oxidizing known quantities of formic acid, or compounds such as glycerol and erythritol which yield known quantities of formic acid when oxidized by lead tetraacetate. Since the carbon dioxide produced is proportional t o the change in pressure, the formic acid content of the unknown solution is determined by reference to the manometer calibration. Procedure 2B. The apparatus is a gas train consisting of the reaction vessel provided with a gas-inlet and gas-outlet tube and a separatory funnel, a dry ice trap, a scrubbing bottle containing concentrated sulfuric acid, and a carbon dioxide adsorption tube containing Ascarite. I n a typical experiment 10 ml. of 90% acetic acid containing 21.5 mg. of formic acid are added to the reaction vessel. The inlet tube is closed and the oxidizing solution, 25 m]. of 90% acetic acid containing 1 gram each of lead tetraacetate and potassium acetate, is introduced through the
separatory funnel. When the reaction has proceeded a t room temperature for 30 to 40 minutes, the inlet tube is opened and a stream of carbon dioxide-free nitrogen is slowly flushed through the apparatus for 30 to 40 minutes, the reaction vessel being shaken owasionally. Carbon dioxide evolved is measured by the increase in weight of the ascarite tube. A blank determination is also made using the lead tetraacetate reagent and 90% acetic acid. ACKNOWLEDGMENT
The technical assistance of J. Giroux is gratefully acknowledged. LITERATURE CITED
(1) (2) (3) (4)
Ahlh, L., and Samuelson, O., ANAL.CHEM.,25, 1263 (1953). Auerbach, F., and Zeglin, H., 2. physik. Chem., 103, 161 (1923). Cordner, J. P., and Pausacker, K. H., J . Chem. SOC.,1953, 102. Grosheints, J. A I , , J . A m . Chem. SOC.,61, 3379 (1939). (5) Rlosher, W. A., and Kehr, C. L., Ibid., 75, 3172 (1953). (6) Perlin, A. S., presented before the Division of Carbohydrate Chemistry at the 124th Meeting of the AMERICAN CHEMICAL SOCIETY,Chicago, Ill. (7) Umbreit, W. W., Burris, R. H., and Stauffer, J. F., “hIanometric Techniques and Tissue Rletabolism,” Minneapolis, Burgess Publishing Co., 1949. (8) Vogel, A. I., “Practical Organic Chemistry,” p. 195, New York, Longmans, Green and Co., 1948. for review December 14, 1953. Accepted February 23, 1954. RECEIVED N.R.C. No. 3276.
Analysis of Alkyd Resins Modified with Vinyl Chloride-Acetate Copolymer MELVIN H. SWANN and GEORGE G. ESPOSITO Paint a n d
W
Chemical Laboratory, Aberdeen Proving Ground, Md.
HEN alkyd resins are modified with vinyl chloride-
acetate copolymer, they dry more rapidly and possess increased chemical and abrasion resistance. As in other types of modified alkyd resins, control is needed on the alkyd resin content as well as on the amount of modifying resin. Analysis for vinyl resins based entirely on total chlorine content does not prevent the substitution of other chlorinated resins and is affected by chlorinated plasticizers and solvents. The alkyd resin content of enamels has always been controlled by analysis for the phthalic anhydride present. All methods for determining phthalic anhydride begin with saponification of the resin with alcoholic potassium hydroxide. Vinyl chloride-acetate resins decompose during saponification and form products tvhich prevent determination of the phthalic anhydride content of the alkyd resin by any existing method. This characteristic of vinyl resins necessitates an analysis based on separation of the vinyl resin from the alkyd. Alkyd resins are soluble in glacial acetic acid and aromatic hydrocarbons and partially soluble in ethyl alcohol, but vinyl chlorideacetate resins are insoluble in these materials.
Use
aromatic for separation give a medium suitable for direct saponification of the alkyd resin, but consistent results could not be obtained in separations with this class of solvents. When the vehicle of the enamel was properly diluted with glacial acetic acid, then added dropwise to an excess of acetic acid, follomd by considerable mechanical agitation, good quantitative separations could be obtained, ex?ept with alkyds modified with raw soybean oil. Ethyl alcohol is an excellent precipitating medium for the separat,ion Of
KO. 1 2
3 4
of the vinyl resins, but alkyd resins are only partially soluble in ethyl alcohol a t room temperature. If alkyd resins are first dissolved in a small amount of dioxane or methyl isobutyl ket’one, they will not separate from solution when poured into alcoholic potassium hydroxide, even a t low concentrations of alkali. Vinyl resins are decomposed by alkali, but a solution of 0.125N potassium hydroxide in absolute ethyl alcohol precipitates the vinyl chloride-acetate resin without decomposition in the time required to effect separation, retaining the alkyd in solut’ion. The separated vinyl resin must then be dried a t a low temperature, since decomposition may occur a t temperatures above 80” C. The alkyd resin is then present in a suitable saponifying medium for further analysis. The vinyl chloride-vinyl acetate copolymers are available in varying proportions of the chloride t o the acetate. A copolymer,
Alkyd Castor oil, modified Soybean oil, modified (low phthalic anhydride) Linseed oil,
Table I. Results of .4nalyses ChlorideAcetate Copolymer Vinyl Chloride by Gravimetric by Chlorine Separation, % Analysis, % Theoretical Found Theoretical Found Vinyl
Phthalic Anhydride, %
Theoretical
Found
32.1
32.5 32.3
29.4
29.5 29.4
23.8
23.8 23.8
33.2
33.2 33.5
30.4
30.1 30.6
15.7
15.7 15.6
modified
34.2
33.1 33.0
31.3
31.6 31.61
...
...
Coconut oil, modified
33.2
34.0 33.5
30.4
31.7 31.7
...
...
